Introduction

Over 600 Middle Eocene bat specimens have been excavated from the Messel pit(Grube Messel, near Darmstadt, Germany), and seven species have been describedthus far. Many of the fossils are preserved as complete skeletons, often withsoft body outlines and gut contents. Six of the bat species represent threeextinct families, whereas Tachypteron franzeni can be assignedto the extant family Emballonuridae (Storch et al., 2002).T. franzeni is known only from two specimens; however,these are extraordinarily well preserved, including the shoulder joints andinner ears, so this had already been recognized in the original description ofT. franzeni, and these close resemblances to extantemballonurids led to the conclusion that T. franzeni hadalready evolved similar bioacoustic specializations and a similar flight styleto modern taxa.

The shoulder joints of bats are sophisticated structures showing remarkablemorphological variation. Miller's (1907) investigations on the differentiationsof the shoulder within the Microchiroptera were continued by the studies ofother authors (Vaughan, 1970; Strickler, 1978; Hermanson and Altenbach,1983).

Three different types of shoulder joint can be distinguished within theChiroptera: the primitive morphology of the shoulder joint with a globularhumeral head and corresponding glenoid cavity, as seen in Megachiroptera andRhinopomatidae; a derived shoulder joint with an oblong humeral head and asingle trough-like articular surface on the scapula, found in members of thesuperfamilies Emballonuroidea, Rhinolophoidea and Noctilionoidea; a derivedshoulder joint with a secondary articulation between the tuberculum majus and asecondary articular facet on the dorsal side of the scapula, as seen in theremaining families. Their distribution within the order gives evidence ofparallel evolution of the derived types (Schlosser-Sturm and Schliemann, 1995).The morphological modifications of the derived joints are interpreted as afunctional response to a biomechanical demand connected with flight (Norberg,2002), i.e., to limit pronation of the humerus during the downstroke of the wingbeat cycle, realized in two different mechanical ways (Schlosser-Sturm, 1982;Altenbach, 1987; Schliemann and Schlosser-Sturm, 1999). Because movementrestriction was described for the primitive type as well (Bergemann, 2003),functional interpretations are still a matter of controversy.